The present invention relates to a strontium containing master alloy and its manufacture and use for the control of the microstructure in aluminum, zinc and magnesium base alloys.
Strontium is known in the art to be a superior and permanent modifier of the aluminum-silicon component of eutectic and hypoeutectic, i.e., less than 12.6 weight percent silicon, aluminum-silicon casting alloys. The addition of strontium modifies the morphology of the eutectic phase to produce a fine, fibrous microstructure, rather than the lamellar or acicular plate-like structure typically encountered in unmodified alloys, thus resulting in an alloy with improved mechanical properties, ductility and impact resistance. Reference should be had, for example, to U.S. Pat. Nos. 3,446,170 and 3,567,429, Canadian Patent 1,829,816, and K. Alker et al. "Experiences with the Permanent Modification of Al--Si Casting Alloys", published in Aluminum, 49(5), 362-367 (1972).
Other alloy systems have found benefits from additions of strontium as well. For example, U.S. Pat. No. 3,926,690 to Morris et al. discloses that the addition of 0.01-0.5% strontium or calcium to an alloy of aluminum-magnesium-silicon provides an alloy with improved extrusion properties. U.S. Pat. No. 4,394,348 to Hardy et al. discloses that the use of a master alloy containing strontium peroxide provided for a finer grain alloy. In "Modification of Intermetallic Phases by Strontium in Aluminum Wrought Alloys", by M. H. Mulzimoglu et al., strontium additions were reported to have a modifying effect on various intermetallic phases of aluminum series alloys 6061, 5182 and 1xxx.
However, there is difficulty involved in the addition of strontium. Strontium is generally added to alloys in the form of a master alloy. The use of pure metallic strontium is limited in that it readily oxidizes in a humid atmosphere and the presence of the oxide layer inhibits the rate of dissolution of the strontium into the desired melt.
In present practice, such strontium additions to alloys are often done utilizing a strontium containing master alloy. Powder compacts containing strontium-silicon are disclosed in U.S. Pat. No. 4,108,646. British Patent 1,520,673 discloses a master alloy of aluminum-silicon-strontium. A strontium-silicon-aluminum master alloy is disclosed in U.S. Pat. No. 4,009,026. U.S. Pat. No. 4,937,044 describes a strontium-magnesium-aluminum master alloy. The majority of strontium-containing master alloys used for modification of aluminum-silicon alloys are manufactured in the form of binary aluminum-strontium master alloys; however, these have disadvantages, and other systems as well have disadvantages.
Thus, for example, the use of these master alloys has always been hindered by slow melting or dissolution rates in low temperature applications. The following illustrative master alloys all reportedly require addition at melt temperatures in excess of 725.degree. C. in order to achieve acceptable dissolution rates and strontium recovery:
(1) master alloy containing 10 weight percent strontium and 90 weight percent aluminum; PA1 (2) master alloy containing 10 weight percent strontium, 14 weight percent silicon and 76 weight percent aluminum; PA1 (3) master alloy containing 90 weight percent strontium and 10 weight percent aluminum; and PA1 (4) master alloy containing 40 weight percent strontium, 35 weight percent aluminum and 25 weight percent magnesium.
In addition, pure metallic strontium, as well as master alloys containing high concentrations of alpha phase strontium, such as 90 weight percent strontium and 10 weight percent aluminum, are very reactive with the atmosphere and require special packaging to prevent oxidation and degradation of the master alloy. This special packaging is usually aluminum which has a liquidus temperature of 660.degree. C., which further hinders the master alloys melting or dissolution rate at lower temperatures.
Many applications utilizing nonferrous alloys operate with the molten metal bath at extremely low temperatures. As an example, molten metal temperatures of 620.degree. C. are common in die casting operations. Also, steel coating lines applying a coating containing 57.5% aluminum, 41% zinc and 1.5% silicon typically operates with a molten metal bath temperature of 600.degree. C. A significant need, therefore, exists in industry for a strontium containing master alloy which would readily melt or dissolve at lower metal temperatures and which is nonreactive and stable in the atmosphere in order to avoid processing difficulties and the necessity for special protective packaging.